In numerous processes described in the patent literature, X and Y zeolitic adsorbents exchanged with various cations are used with certain desorbents to separate the para-isomer of dialkyl-substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. For example, in Neuzil et al U.S. Pat. No. 3,686,342, para-diethylbenzene is the preferred desorbent. More recently, in Zinnen U.S. Pat. Nos. 4,886,930; 4,864,069; 5,012,038 and U.S. Pat. No. 5,057,643, other "heavy" desorbents, e.g., diethyltoluene and tetralin and tetralin derivatives, have been disclosed for a process for separating para-xylene where the feed mixtures contain higher boiling aromatic hydrocarbons such as C.sub.9 aromatics, C.sub.10 aromatics, etc. With these C.sub.9 and higher aromatic impurities in the feed, it is difficult to separate the desorbent, p-DEB, from the C.sub.9.sup.+ aromatics by fractionation because the boiling points of these materials are so close. If not removed, the C.sub.9 aromatics would gradually build up in the desorbent, which must be recycled to the separation process for economic reasons.
U.S. Pat. No. 4,864,069 to Zinnen discloses a process for separating para-xylene from a mixture of xylene isomers with diethyltoluene as a heavy desorbent in which the heavy desorbent is recovered and recycled to the process by fractionating the raffinate to separate the desorbent from the xylene isomers and C.sub.9 impurities. Likewise, after the para-xylene is removed from the extract in the extract column, the bottoms of the extract column (fractionation) contain desorbent and C.sub.9 aromatic impurities which is further fractionated to recover the desorbent for return to the adsorbent separation process. In a co-pending application, Ser. No. 791,697, filed Nov. 14, 1991 the step of fractionating the extract bottoms to remove C.sub.9 aromatic impurities from the desorbent recycle stream was eliminated by removing a portion of the extract bottoms stream and diverting the portion to the raffinate fractionation column where C.sub.9 aromatics in the extract bottoms stream are separated from the desorbent, which is recycled to the separation step. In the present invention, the amount of desorbent in the C.sub.9 aromatics sent to the raffinate column from the extract column is reduced, thereby lowering the energy input to the raffinate column and also reducing the raffinate column size and cost.
In U.S. Pat. Nos. 4,886,930 and 5,057,643 to Zinnen, a heavy desorbent, tetralin and derivatives, were disclosed for use in the separation of para-xylene from mixtures of xylene isomers. The recovery of the heavy desorbent from raffinate and extract streams by simple fractionation is also contemplated there.
It is also known that crystalline aluminosilicates, i.e., or zeolites, are used in adsorption separations of various mixtures in the form of agglomerates having high physical strength and attrition resistance. Methods for forming the crystalline powders into such agglomerates include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to the high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. As binders, clays of the kaolin type, water permeable organic polymers or silica are generally used.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The invention may also be practiced in a cocurrent, pulsed batch process, like that described in U.S. Pat. No. 4,159,284 or in a cocurrent, pulsed continuous process, like that disclosed in Gerhold U.S. Pat. Nos. 4,402,832 and 4,478,721.
The functions and properties of adsorbent and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al U.S. Pat. No. 4,642,397 is incorporated herein.
We have discovered a process for separating para-xylene from its isomers including ethylbenzene employing a zeolite adsorbent and a heavy desorbent whereby substantial economies can be obtained by the reduction of the quantity of desorbent required to be processed in the raffinate fractionation column. In this process, a sidecut stream comprising all of the C.sub.9 aromatics in the extract is taken from the extract fractionation column. The sidecut stream contains from 10 to 20 percent of the desorbent charged to the extract column and is directed to the raffinate fractionation column to recover the desorbent. Since the sidecut removes all C.sub.9 aromatics from the extract stream, C.sub.9 aromatics are eliminated from the extract bottoms desorbent recycle stream.